Automatic frequency control for a sandwich transducer using voltage feedback

ABSTRACT

An automatic drive control circuit for energizing or driving a sandwich transducer, which includes a mechanical displacement amplifier, piezoelectric crystal drivers and feedback piezoelectric crystals, compares the feedback voltage proportional to displacement and the driving voltage. The frequency of such driving voltage is automatically adjusted to drive the sandwich transducer at mechanical resonance, which occurs when the feedback voltage equals or exceeds the driving voltage. The drive control includes a sweep circuit coupled to a variable frequency oscillator, which produces an AC square wave signal at a frequency dependent on the sweep signal, and the AC signal is amplified and provided as the driving signal to the sandwich transducer. A comparator compares the driving voltage with the feedback voltage, and when the latter equals or exceeds the former, the sweep circuit locks at a fixed voltage to lock the oscillator AC signal frequency, which is maintained constant while the sandwich transducer is maintained at mechanical resonance.

United States Patent 11 1 1111 3,889,166

Scurlock 1 June 10, 1975 AUTOMATIC FREQUENCY CONTROL FOR 57 ABSTRACT A SANDWICH TRANSDUCER USING An automatic drive control circuit for ener i VOLTAGE FEEDBACK g Zmg driving a sandwich transducer, which includes a meinveniofi Lawrence Scllrlock, yr Ohio chanical displacement amplifier, piezoelectric crystal drivers and feedback piezoelectric crystals, compares [73] Asslgnee' Qumtmn Columbus Ohio the feedback voltage proportional to displacement and [22] Filed: Jan. 15, 1974 the driving voltage. The frequency of such driving [211 App] No 433 575 voltage is automatically adjusted to drive the sandwich transducer at mechanical resonance, which occurs when the feedback voltage equals or exceeds the drivl /8-3; ing voltage. The drive control includes a sweep circuit 310/26 coupled to a variable frequency oscillator, which pro- [51] Int. Cl. H02n 3/00; H02n 5/00; H02n 7/00 duces an AC square wave signal at a frequency depenl Field of Search S dent on the sweep signal, and the AC signal is amplil3l/.5 fied and provided as the driving signal to the sandwich transducer. A comparator compares the driving volt- [56] References Cited age with the feedback voltage, and when the latter UNITED STATES PATENTS equals or exceeds the former, the sweep circuit locks 3 434,074 3/1969 Libby 318/118 at a fixed voltage to lock the Oscillator AC Signal 3:539,888 11/1970 Prisco et a1. 318/116 q y which is maintained Constant while the and- 3,578,996 5/1971 Balamuth 310/8.7 h transducer is maintained at mechanical reso- 3,586,936 6/1971 McLeroy 310/8.1 nance.

3,716,828 2/1973 Massa 340/10 3,819,961 6/1974 Bourgeois et a1. 3l0/8.1

Primary ExaminerMaynard R. Wilbur Assistant Examiner-T. M. Blum 11 Claims, 3 Drawing Figures Attorney, Agent, or FirmFrank H. Foster omvs CONTROL---1\ /1 AMPLIFIER 40 SANDWICH [OSCILLATOR P43 1 I 1? T, TRANSDUCER i J SWEEP VOLTAGE 20 cmcuur COMPARATOR I L 471 AUTOMATIC FREQUENCY CONTROL FOR A SANDWICH TRANSDUCER USING VOLTAGE FEEDBACK BACKGROUND OF THE INVENTION This invention relates to an automatic drive control circuit for a mechanical displacement device, and more particularly to such a drive control that provides automatic frequency control of a sandwich transducer, for example, in the form of a power driven osteotome by using voltage feedback to maintain mechanical resonance.

One technique for maintaining mechanical resonance in a sandwich transducer, which includes a mechanical displacement amplifier having, for'example, piezoelectric crystal drivers maintained under compression therein, has been to control manually the frequency of the drive signal applied to the piezoelectric crystals. One drawback to this technique is that the resonant frequency of the sandwich transducer may vary with the applied load, thus requiring frequent frequency adjustments. Another standard method for controlling automatically the drive signal frequency has been to make an electrical phase comparison of the voltage and current of the drive signal, whereby when such voltage and current are in phase, mechanical resonance is achieved, but such controls are often complex, expensive and inefficient.

An example of a sandwich transducer in which piezoelectric crystals maintained under compression are used to drive a mechanical displacement amplifier used as an osteotome is described in a patent application of Wootten and Scurlock for Power Driven Osteotome", Ser. No. 391,341, filed Aug. 24, 1973, which patent application is assigned to the same assignee as the instant patent application. In the referenced sandwich transducer a drive signal preferably in the ultrasonic frequency range and at a frequency to maintain mechanical resonant operation efficiency is provided to piezoelectric drivers to effect mechanical displacement in the instrument, and such displacement is amplified at a cutting tip output portion, which has a reduced crosssectional area relative to that of the major extent of the sandwich transducer.

SUMMARY OF THE INVENTION The drive control provides a drive signal to a pair of piezoelectric crystals in a sandwich transducer to effect displacement therein, and a further paid of piezoelectric crystals in the sandwhich transducer monitors such displacement and generates a feedback voltage having an amplitude proportional thereto. An AC signal preferably in the form of a square wave is generated by an oscillator and amplified by an amplifier for application as the drive signal to the driving piezoelectric crystals, and a sweep signal, which is preferably a ramp signal, from a sweep circuit determines the frequency of the AC signal. A voltage comparator compares the drive and feedback voltages and generates either a first output to maintain sweeping in the sweep circuit, when the former exceeds the latter voltage, or generates a second output to lock the sweep circuit voltage at a level which maintains the oscillator AC signal frequency constant when the feedback voltage equals or exceeds the driving voltage, which is indicative of mechanical resonance in the sandwich transducer. Moreover, mechanical displacement of the sandwich transducer for determining the point at which the oscillator frequency is locked is a primary feature of the invention and can be effected using, for example, the preferred circuit embodiment described hereinafter or using other techniques to determine resonant instrument operation, such as by using a peak signal detector or phase detector responsive to the feedback signal generated in the sandwich transducer. Also mechanical displacement is proportional to power input, and by varying the drive signal power when the sandwich transducer is maintained at resonance the amount of mechanical displacement can be adjusted.

With the foregoing in mind, it is a principal object of this invention to drive efficiently a mechanical displacement device at resonance.

Another object is to control automatically the drive signal to a mechanical displacement amplifier for effecting and maintaining resonant operation thereof.

A further object of the invention is to provide automatic frequency control of a sandwich transducer to maintain mechanical resonance thereof using a feedback voltage comparison signal proportional to the mechanical displacement of the sandwich transducer.

An additional object of the invention is to provide an automatic drive control improved in the noted respects.

These and other objects and advantages of the present invention will become apparent as the following description proceeds.

To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims, the following description and the annexed drawings setting forth in detail a certain illustrative embodiment of the invention, this being indicative, however, of but one of the various ways in which the principles of the invention may be employed.

BRIEF DESCRIPTION OF THE DRAWINGS In the annexed drawings:

FIG. 1 is an elevational view with portions cut away of a sandwich transducer coupled to a drive control;

FIG. 2 is an electrical block diagram of a drive control for providing a drive signal to a sandwich transducer and receiving a feedback signal from the latter; and

FIG. 3 is a schematic electric circuit diagram of the automatic drive control of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, wherein like reference numerals designate like parts in the several figures, and particularly to FIG. 1, a drive control 1 is illustrated connected to provide a drive signal to a sandwich transducer generally indicated at 2. The sandwich transducer has a main body portion 3 and an output tip portion 4 of reduced cross-section relative to the latter for displacement amplification, as is known in the art. Within the main body portion is included a cylindrical forward mass or handle 5, an electro-mechanical energy conversion device generally indicated at 6, a cylindrical rearward mass or matching stub 7 and a connecting rod 8, which couples the former elements along a longitudinal axis while maintaining compression of the electro-mechanical energy conversion device.

The rearward end of the handle 5 has a threaded opening 9 tapped therein with the interior'transverse surface thereof being substantially flat, and the forward end of the connecting rod 8 is threaded to its termination in a substantially completely flat surface to abut the former surface at 10 in a bottom-out relationship for optimum transmissibility within the device. Moreover, the matching stub 7 is also interiorly threaded at 11 along the longitudinaL-axis thereof for coupling to the rearward threaded portion of the connecting rod, and a torquing force may be applied to the former at the recesses 12 in the rearward transverse surface 13 thereof to maintain the main body portion 3 of the sandwich transducer under compression along the connecting rod.

The electro-mechanical energy conversion device 6 includes, for example, two polarized driving piezoelectric crystals 14, 15 and an electrically conductive shim electrode 16 positioned therebetween, which are slid into position circumferentially over a cylindrical insulator 17 surrounding a smooth portion 18 of the connection rod 8 in orientation with the positive crystal surfaces in connection with the shim. Moreover, two additional feedback polarized piezoelectric crystals 19, 20 or other device for producing a feedback voltage representative of displacement in the transducer 2 and electrically conductive shim electrode 21 therebetween are also similarly positioned relative to theinsulator l7 and connecting rod 8 as shown. The longitudinal length of the insulator 17 may be slightly less than the longitudinal thickness of the energy conversion device 6 to avoid interference with abutment of the corresponding surfaces of the latter with respective rearward and forward transverse surfaces of the handle and matching stub; or alternatively the insulator length may be the same or greater than the thicknessof the energy conversion device, and in the latter case the handle and matching stub would be counter-bored to accommodate the same. y

A further electrically conductive shim electrode 22 is located between opposed surfaces of the piezoelectric crystals 15, 19 and a wire 22a coupled to such electrode is connected to a grounding terminal 23 of the sandwich transducer, which terminal is also coupled by a wire to a grounding connection 24 for the sandwich transducer. Shim electrodes 16, 21 are connected by lines 16a, 21a to respective terminals 25, 26 in a conventional strain relief fitting 27 for subsequent connection by respective lines 23a, 25a, 26a to the drive control 1. Moreover, the strain relief fitting 27 is positioned in sealed engagement with a cap portion 28 covering the matching stub 7 and electromechanical energy conversion device 6. Such cap portion is also connected in sealed engagement with a flange 29 formed on the handle portion 5 and facilitates sterilization of the instrument 2 without wetting the energy conversion device.

The sandwich transducer 2 provides a stepped horn mechanical displacement amplifier effect, whereby the oscillatory wave produced by the energy conversion device 6 is amplified as to displacement at the output end 30 of the output tip portion 4, and preferably maximum transmissibility of the forces and displacement in the instrument with a minimum electric power input is achieved by having all connections between adjacent elements being at substantially completely flat abutting connections with the instrument being driven atresonance. Moreover, the distance from the conductive shim 22 to the rearward transverse surface 13 is on the order of onequarter wavelength of the oscillatory wave in the instrument when driven approximately at resonance, and the distance from such conductive shim to the forward transverse surface 31of the le portion 5 is on the order of one-half waveiengm. lne length of the output tip portion 4 from the forward transverse surface 31 of the handle to the output end 30 is approximately one-quarter wavelength. Thus, the oscillatory wave produced in the instrument by the energy conversion device 6 develops a minimum displacement at the latter and at the forward end of the handle and a maximum amplified displacement at the output end 30.

In operation of the sandwich transducer 2, an ultrasonic input signal from the drive control 1 to the drive piezoelectric crystals 14, 15 effects production of an oscillatory wave in the sandwich transducer, The frequency ofthe drive signal is automatically varied in the drive control 1 in response to a feedback voltage generated by the feedback piezoelectric crystals 19, 20, which voltage is proportional to the mechanical displacement of the sandwich transducer, so that the wavelength of the oscillatory wave is approximately equal to the longitudinal. length of the total sandwich transducer from the output end 30 to the rearward transverse surface 13 of the matching stub. Moreover, the oscillatory wave is provided at the output end 30 as a displacement forceyand although the stroke distance and force of each displacement motionmay be relatively small, the sum of rapidly occurring displacements is effective to cut or to shave bone as an osteotome, to drive a pin or brace or to drill a hole in bone, etc. The sandwich transducer shown and described is known as a full-wavelength instrument; although the drive control of the invention may be used to drive sandwich transducers being any integral number of wavelengths over its longitudinal dimension.

Turning now more particularly to FIG. 2, the drive control 1 is shown divided into several portions to pro. vide a drive signal on the line 16a to t2, drive piezoelectric crystals 14, 15 in the sandwich transducer 2, which generates a mechanical displacement indicated by the arrow 40. The feedback voltage from the feedback piezoelectric crystals 19, 20 in the sandwich transducer is applied to'the drive control via line 21a, and a grounding connection is shown at 41 for the conductive shim 22 and the surfaces of the piezoelectric crystals 14, 20, which are normally in abutment with surfaces of the handle portion 5 and matching stub 7 of the sandwich transducer.

The drive control 1 includes a sweep circuit 42 that produces a sweep signal preferably in the form of a ramp signal to control the frequency of an AC signal generated by a variable frequency oscillator43. Thus, as the sweep signal varies in magnitude, the frequency of the oscillator output AC signal varies proportionally thereto. Moreover, an amplifier 44 coupled to the oscillator 43 amplifiesthe AC signal and applies the same on line 25a to the driving piezoelectric signals 14, 15 in the sandwich transducer 2. I

A voltage comparator 45 has a first input 46 connected to receive a reference voltage representative of thevoltage of the drive signalfrom the amplifier 44 and a second input 47 coupled to the line 21a to receive the feedbackvoltage from the feedbackpiezoelectric crystals 19, 20. Withinthe .voltage comparator 45 the drive signal voltage, as represented by the noted reference voltage, and feedback voltages are compared, and when the latter equals orexceeds the former, which occurs when the sandwich transducer 2 is driven at resonance, the voltage comparatoreffects locking of the sweep. circuit at the. instantportion of the sweep signal voltage thereof,.and the oscillator frequency is thereby maintained constant atthat point. Moreover, such oscillator frequency remains fixed for resonant .frequency operation of: the sandwich transducer2- so longas the feedback voltage equals :orv exceeds the drive signal voltage.

Although the several portions of the drive control 1 aredescribed in detail below, such portions may alternatively be other types .of conventional voltage comparators, sweep circuits, oscillators and amplifiers. For example, the voltage comparator 45 may be a conventional devicewhich compares twouvoltages and provides at its output respective indications of which input voltage is the larger. Moreover, the sweep circuit 42 may be a conventional r-amp signal generator which periodically generates a .ramp signal when the voltage comparator produces a first output and locks at a fixed voltage when the latter produces a second output; and the oscillator; 43 may also be a conventional device which provides an output AC signal preferably in the ultrasonic frequency range and at a frequency determined by the input signal'thereto. Similarly, the amplifier 44 may be a conventional AC amplifier device to amplify the oscillator output AC signal and to provide the same in amplified form as a drive signal'input to the sandwich 'transducert2. t

Referring now specifically to FIG. 3, the preferred form of the drive control 1 is illustrated in detail, including the sweep circuit portion 42, variable frequency oscillator portion 43, amplifier portion 44 and voltage comparator portion 45. A dashed line la surrounds those -circuit parts which are preferably mounted on a printed circuitboard, and connections from thelatter to power dissipating parts mounted, for example, on a chassis, not shown, are indicated by dots throughout the circuit. Moreover, fixed and relatively variable DC voltage supply circuit portions 50, 51 are also included in the'drive control for supplying, respectively, a relatively fixed voltage'for use as a bias voltage and a relatively variable voltage for-varying the power supplied to the driving piezoelectric crystals 14, for adjustment'of the displacement magnitude in the sandwich transducer when driven at resonance.-

The drive control lreceives 120 volt, 60 Hz line voltage, for example from the utilities company, on the power lines 52, 53, which are connected both to the fixed and variable voltage power supplies 50, 51, the

a coupling transformer 56." Moreover, a grounding connection for the drive control and piezoelectric crystals is provided on the line'23ab t w The variablevolta'ge power supply portion 1 includes a conventional adjustable transfor'mer 57 having an adjustable arm 58 to provide from'zero to full line voltage to the primary winding of a step down transformer 59. The secondary winding of the step down transformer 59 is connected to a full wave bridge rectifier 60 having an output connected across a storage capacitor 61 to provide an adjustable DC voltage across the lines 62, 23a, which voltage may be varied, for examplegfrom 0 to 40 volts by adjustment of the arm 58. The line 62 is connected to the amplifier and voltage comparator portions 44,45 by the lines 63, 64, respectively, to provide the above-noted reference voltage.

A first isolation circuit 65 in the voltage comparator 45 includes a variable resistance circuit 66 for calibration, transistors 67, 68, resistor 69 and diode 70 for isolating the signal on the line 64 from the other portions of the voltage comparator. Thus, the voltage appearing at the collector. of the transistor 67 is proportional to the voltage on the lines 62, 64 from the variable voltage power supply 51 which represents the magnitude of the AC signal applied by the coupling transformer 55 to the driving piezoelectric crystals 14, 15. A test terminal 67a coupled to the collector of the transistor 67 is provided for convenient monitoring of the signals appearing thereat, and other similar test terminals are provided at various parts of the drive control as illustrated.

The coupling transformer 56 connected to the feedback piezoelectric crystals 19, 20 forms part of the voltage comparator 45 and the secondary of such transformer is connected across a full wave bridge rectifier 71 having an output connected across a storage capacitor 72 to provide a feedback DC voltage representative of the amount of displacement in the sandwich transducer 2 via the line 73 to a further isolation circuit 74. The latter includes a variable resistance circuit 75 for calibration, transistors 76, 77, resistor 78, and diode 79, all coupled similarly to those elements in the first isolation circuit 65. Thus, the voltage appearing at the collector of the transistor 76 is proportional to the feedback voltage and displacement in the sandwich transducer 2.-A comparison circuit 80 in the voltage comparator 45 includes a transistor 81, diodes 82, 83 and resistors 84., 85 for comparison of the outputs of the isolation circuits 65, 74. Thus, whenever the signal representing the drive signal voltage, which is applied to the emitter of the transistor 81, exceeds the signal representing the feedback voltage, which is applied to the base of such-transistor, the transistor is biased to conduction'and a signal is applied at the collector thereof on the line 86 at the control input 87 to the sweep circuit 42. Moreover, whenever the feedback voltage equals or exceeds the drive signal voltage, the transistor 81 is biased off, and no signal is applied on the line 86 to the sweep circuit effecting locking of the same, as will be described in more detail below.

The fixed voltage power supply 50, includes a transformer 90 having a primary winding coupled to the drive control input power lines 52, 53 and a secondary winding connected across a full wave bridge rectifier 91 having an output coupled across a storage capacitor 92 to provide a DC voltage input to a voltage regulator circuit 93. The latter including transistors 94, 95, resistors 96, 97, diodes 98, 99 and zener diode 100 is effective to provide a highly regulated DC voltage at the node or point 101, which is used throughout the drive control to provide a bias voltage for the various circuit portions therein.

The sweep circuit 42 is connected to the bias voltage point 101 by the line 101a and receives a sweep control signal at its control input 87 on the line 86 from the voltage comparator 45 via the resistors 102, 103. A transistor 104 receives such sweep control signal and such transistor becomes either conductive or nonconductive, respectively depending on whether the transistor 91 in the voltage comparator is conductive or non-conductive. Transistor 105, zener diode 106, resistors 107, 108 and potentiometer 109 form a constant current source for a substantially linearly charging voltage across a capacitor 110 in a conventional unijunction transistor 111 oscillator.

Moreover, the voltage at zener diode 112 determines or sets a minimum voltage on the capacitor shortening the maximum sweep time of the sweep circuit, during which the capacitor 110 is charged. A line 113 from the output of the capacitor 110 and unijunction transistor 111 oscillator is connected to the input of an amplifier 114, which provides a signal via a diode 115 to a node or point 116 at an input of the variable frequency oscillator 43. Also, the voltage across a zener diode 117 is provided via a diode 118 to the point 116, and such zener diode thus sets the minimum output voltage of the sweep circuit to determine a minimum AC signal frequency from the oscillator 43.

in operation of the sweep circuit portion 42 of the drive control, conduction in the input transistor 104 which occurs when the drive signal exceeds the feedback voltage, allows the capacitor 110 to charge periodically via the constant current circuit and to be discharged periodically via the unijunction transistor 111. As the voltage on the capacitor 110 increases, the voltage at the output of the amplifier 114 similarly increases and is applied as a ramp or sweep signal via the diode 115 to the input at node 116 of the variable frequency oscillator 43. The sweep time for the sweep circuit 42 is determined by the time required for the capacitor 110 to charge to the threshold level of unijunction transistor 111, which then discharges the same, and for added efficiency the sweep time is shortened by the zener diode 112, which provides a signal via a diode 112a to charge rapidly the capacitor at least to a minimum level after each time the latter has been discharged. Moreover, whenever the sweep control signal output of the voltage comparator 45 ceases, which occurs when the feedback voltage equals or exceeds the drive signal voltage, the transistors 81, 104 become non-conductive, and the capacitor 110 locks onto its instantaneous voltage and maintains the same for amplification by the high impedance amplifier 114.

The variable frequency oscillator 43 includes a conventional unijunction transistor 120 and capacitor 121 oscillator, and a charging circuit for the capacitor includes a resistor 122 and potentiometer 123. One base electrode of the unijunction transistor 120 is connected by a resistor 124 to the sweep circuit input at point 116 and the voltage at the latter determines the level to which the capacitor 121 must charge before reaching the threshold level of the unijunction transistor which then fires to discharge the same. Thus, the frequency of the signal appearing at the output 125 of the unijunction transistor oscillator is approximately inversely proportional to the voltage appearing at the point or node to a pulse shaping circuit 127.

The purpose of the pulse shaping circuit 127, which includes first and second integrated circuits 128, 129 having a bias voltage applied from the zener diode and capacitor 130 on the line 131, 132, is to shape accurately the oscillating signal input applied on line 126 to a well defined square wave. Moreover, such circuit provides the shaped square wave signal on.- line 133 to the base input of transistor 134 at the collector output of which the oscillator output AC signal is provided via line 135 to the input 136 of the amplifier 44.

The first integrated circuit 128 includes two switching NAND gates 137, 138 and two inverting feedback delay NAND gates 139, 140, the former two being operative to provide output signals having rapid rise and fall times on lines 137a, 138a, respectively, to the set and reset-inputs of a conventional .IK flip-flop 141 in the second integrated circuit 129. The NAND gates produce a high level output when at least one input thereof is at low level and produce a low level output when both inputs thereof are at high level. A connection from the 0 output of the flip flop 141 via the line 142 to the input of the inverting NAND gates 139, provides a signal which operates through delay capacitor 143 and line 144 as an input to the switching NAND gate 137 to enable the same to change logic level when an appropriate highsignal is received via the line 26. Moreover, a connection from the 6 output of the flipflop 141 via the line 145 to one of the inputs of the switching NAND gate 138 is similarly used to provide an enable signal for the same. Operation of the NAND gates and flip-flop in the respective integrated circuits 128, 129 may be easily traced dout following conventional logic circuit principles. Essentially, however, the integrated circuit 128 converts the oscillating signal received on line 126 to a proper logic level and pulse shape for operating the flip-flop 141 in the integrated circuit 129 with one input of the flip-flop alternately being blanked by virtue of the feedback lines 142, 145. Moreover, for each oscillating signal pulse on line 126, the flip-flop 141 changes logic level at its Q output, and such signal at a frequency half that of the oscillating signal on line 126 is applied via line 133 to control the transistor 134.

The oscillator 43 output AC signal is applied via the line 135 to the input 136 of the amplifier 44, which not only amplifies the same, but also provides DC to AC signal conversion by virtue of the amplifier connection to the center-tapped primary winding of the transformer 55. More specifically, an amplifier stage 44a of the amplifier includes a transistor 150, which receives collector power from the fixed DC voltage supply 50 via a resistor 15.1 and line 152, and transistor 153, 154, the former operating asa signal inverter and the latter receiving collector power from the fixed DC voltage supply 50 via a resistor and line 156.

Moreover, the DC to AC conversion portion 44b of the amplifier 44 includes control transistor 157,158,

which drive respective power transistors 1'59, 160. The

respective collectors of power transistors 159 160, are connected to opposite ends of the'primai y winding of coupling transformer 55 to provide for current flow therethrough in opposite directions when respectively energized to conduction. Moreover, diodes 161, 162 are also coupled to such respective collectors to conduct residual currents maintained by the inductance of tors159, 160 are cut off.

In operation of the amplifier 44, whenever the AC signal on line 135 is positive, transistor 150 is biased on to conduct any signal on line 152 to the drive control ground line 23a, and power transistor 159 will be cut off. Moreover, transistor 153 will also be biased on, which cuts off transistor 154 to permit the signal on line 156 to bias power transistor 160 to conduction, which closes a circuit for current to flow from the variable voltage power supply 51 downwardly through the lower half of center-tapped primary of transformer 55. Such current flows from line 62 via line 63, downwardly through the lower half of the primary winding of transformer 55 through power transistor 160 to ground line 23a. Such transformer energization provides at its secondary winding one-half an AC drive signal on line 16a to the drive piezoelectric crystals l4, 15.

When the AC signal on line 135 goes negative to relative ground potential through transistor 134, transistor 153 will be cut off, transistor 154 will conduct, and power transistor 160 will be cut off. Moreover, transistor 150 will be cut off, and power transistor 159 will be biased on to conduct current from variable voltage power supply line 62 via line 63 upwardly through the upper half of center-tapped transformer 55 primary winding through transistor 159 to the circuit ground line 23a providing the other half of AC drive signal to the crystals. Thus, the current flowing through the transformer 55 primary alternately in opposite directions induces an AC signal in the transformer secondary to drive the piezoelectric crystals 14, to effect operation of the sandwich transducer 2 normally at resonance.

It should now be clear that the drive control 1 provides a drive signal to drive a sandwich transducer, and a feedback signal indicative of displacement in the latter controls the former to adjust automatically the drive signal frequency to effect mechanical resonant sandwich transducer operation.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An automatic drive circuit for driving a mechanical displacement device or the like at resonance, comprising in combination:

a. variable frequency oscillator means for generating an AC signal at a frequency determined by an input signal thereto and in a range including the resonant frequency of such device;

b. amplifier means responsive to such AC signal for producing a drive signal for said mechanical displacement device to effect displacement in the same;

c. feedback means mounted to said device and responsive to such displacement for producing a feedback voltage proportionally representative of said displacement;

comparator means for comparing the voltage of such drive signal and such feedback voltage, said comparator means producing a first sweep control output signal when said drive signal voltage exceeds such feedback voltage, and producing a second sweep control output signal when said feedback signal exceeds said drive signal;

e. sweep circuit means having an input connected to the output of said comparator means and responsive to such sweep control output signal for generating a sweep signal as a periodcially varying voltage signal in response to said first sweep control output signal, when'such drive signal voltage exceeds such feedback voltage, and for generating a fixed voltage output signal, when such feedback voltage equals or exceeds such drive signal voltage, said sweep circuit means having an output coupled to said variable frequency oscillator means to provide such sweep signal as such input signal to said variable frequency oscillator;

whereby said voltage comparator means locks said sweep circuit means to produce a fixed voltage output signal at a magnitude to effect operation of said variable frequency oscillator means and amplifier means at a fixed frequency to provide a drive signal to the mechanical displacement device for driving the same at resonance when such feedback voltage equals or exceeds such drive signal voltage and whereby said voltage comparator unlocks said sweep circuit means to produce said periodically varying voltage signal to drive said variable frequency oscillator as a sweep frequency oscillator when said drive signal voltage exceeds said feedback voltage.

2. An automatic drive circuit for driving a mechanical displacement device or the like at resonance, comprising in combination:

variable frequency oscillator means for generating an AC signal at a frequency determined by an input signal thereto; amplifier means responsive to such AC signal for producing a drive signal for said mechanical displacement device to effect displacement in the same;

feedback means responsive to such displacement for producing a feedback voltage proportionally representative thereof; comparator means for comparing the voltage of such drive signal and such feedback voltage, said comparator means producing a sweep control output signal indicative of whether such drive signal voltage exceeds such feedback voltage; sweep circuit means responsive to such sweep control output signal for generating a sweep signal as a periodically varying voltage signal, when such drive signal voltage exceeds such feedback voltage, and as a fixed voltage signal, when such feedback voltage equals or exceeds such drive signal voltage, said sweep circuit means being coupled to said variable frequency oscillator means to provide such sweep signal as such input signal thereto;

whereby said voltage comparator means locks said sweep circuit means to produce such fixed voltage sweep signal at a magnitude to effect operation of said variable frequency oscillator means and amplifier means to provide a drive signal to the mechanical displacement device for driving the same at resonance when such feedback voltage equals or exceeds such drive signal voltage wherein said variable frequency oscillator means comprises a pulse shaping circuit.

3. An automatic drive circuit for driving a mechanical displacement device or the like at resonance as set forth in claim 2,

wherein said pulse circuit comprises a plurality of NAND circuits coupled to a JK flip-flop circuit.

4. An automatic drive circuit for driving a mechanical displacement device or the like at resonance, comprising in combination:

variable frequency oscillator means for generating an AC signal at a frequency determined by an input signal thereto; 7

transistorized amplifier means responsive to such AC signal for producing a drive signal for said mechanical displacement device to effect displacement in the same;

feedback means responsive to such displacement for producing a feedback voltage proportionally representative thereof;

comparator means for comparing the voltage of such drive signal and such feedback voltage, said comparator means producing a sweep control output signal indicative of whether such drive signal voltage exceeds such feedback voltage;

sweep circuit means responsive to such sweep control output signal for generating a sweep signal as a periodically varying voltage signal, when such drive signal voltage exceeds such feedback voltage, and as a fixed voltage signal, when such feedback voltage equals or exceeds such drive signal voltage, said sweep circuit means being coupled to said variable frequency oscillator means to provide such sweep signal as such input signal thereto;

whereby said voltage comparator means locks said sweep circuit means to produce such fixed voltage sweep signal at a magnitude to effect operation of said variable frequency oscillator means and amplifier means to provide a drive signal to the mechanical displacement device for driving the same at resonance when such feedback voltage equals or exceeds such drive signal voltage wherein said amplifier means further comprises a coupling transformer coupled between the same and said mechanical displacement device, said coupling transformer including a center-tapped primary winding for periodic and alternate current flow in opposed directions through at least portions of the same, the secondary of said amplifier being coupled to provide such drive signal to drive the amplifier means responsive to such AC signal for producing a drive signal for said mechanical displacement device to effect displacement in the same;

feedback means responsive to such displacement for producing a feedback voltage proportionally representative thereof;

comparator means for comparing the voltageof such drive signal and such feedback voltage, said comparator means producing a sweep control output signal indicative of whether such drive signal voltage exceeds such feedback voltage; k

sweep circuit means responsive to such sweep control output signal for generating a sweep signal as a periodically varying voltage signal, when such drive signal voltage exceeds such feedback voltage, and as a fixed voltage signal, when such feedback voltage equals or exceeds such drive signal voltage, said sweep circuit means being coupled to said variable frequency oscillator means to provide such sweep signal as such input signal thereto;

whereby said voltage comparator means locks said wherein said comparator means comprises a coupling transformer having a primary winding connected for energization by said piezoelectric crystals in said feedback means, and a full-wave bridge rectifier coupled to the secondary winding of said coupling transformer for providing DC feedback.

7. An automatic drive circuit for driving a mechanical displacement device or the like at resonance, comprising in combination:

variable frequency oscillator means for generating an AC signal at a frequency determined by an input signal thereto;

amplifier means responsive to such AC signal for producing a drive signal for said mechanical displacement device to effect displacement in the same;

feedback means responsive to such displacement for producing a feedback voltage proportionally representative thereof;

comparator means for comparing the voltage of such drive signal and such feedback voltage, said comparator means producing a sweep control output signal indicative of whether such drive signal voltage exceeds such feedback voltage;

sweep circuit means responsive to such sweep control output signal for generating a sweep signal as a periodically varying voltage signal, when such drive signal voltage exceeds such feedback voltage, and as a fixed voltage signal, when such feedback voltage equals or exceeds such drive signal voltage, said sweep circuit means being coupled to said variable frequency oscillator means to provide such sweep signal as such input signal thereto;

whereby said voltage comparator means locks said sweep circuit means to produce such fixed voltage sweep signal at a magnitude to effect operation of said variable frequency oscillator means and amplifier means to provide a drive signal to the mechanical displacement device for driving the same at resonance when such feedback voltage equals or exceeds such drive signal voltage wherein said comparator means comprises first and second isolation circuits and a comparison circuit, said first isolation circuit receiving such drive signal voltage and providing a first output signal representative thereof and said second isolation circuit receiving such feedback voltage and producing a second output signal representative thereof, said comparison circuit comparing said first and second output signals and providing said sweep control output signal as a high level signal when the former exceeds the latter and as a low level signal when the latter equals or exceeds the former.

8. An automatic drive circuit for driving a mechanical displacement device or the like at resonance, comprising in combination:

variable frequency oscillator means for generating an AC signal at a frequency determined by an input signal thereto; amplifier means responsive to such AC signal for producing a drive signal for said mechanical displacement device to effect displacement in the same;

feedback means responsive to such displacement for producing a feedback voltage proportionally representative thereof; comparator means for comparing the voltage of such drive signal and such feedback voltage, said comparator means producing a sweep control output signal indicative of whether such drive signal voltage exceeds such feedback voltage; sweep circuit means responsive to such sweep control output signal for generating a sweep signal as a periodically varying voltage signal, when such drive signal voltage exceeds such feedback voltage, and as a fixed voltage signal, when such feedback voltage equals or exceeds such drive signal voltage, said sweep circuit means being coupled to said variable frequency oscillator means to provide such sweep signal as such input signal thereto;

whereby said voltage comparator means locks said sweep circuit means to produce such fixed voltage sweep signal at a magnitude to effect operation of said variable frequency oscillator means and amplifier means to provide a drive signal to the mechanical displacement device for driving the same at resonance when such feedback voltage equals or exceeds such drive signal voltage wherein said sweep circuit means comprises an oscillator circuit including a charging capacitor and controlled means for substantially linearly charging said charging capacitor, said controlled means being responsive to such sweep control output signal to effect charging of said charging capacitor when such sweep control output signal indicates that such drive signal voltage exceeds such feedback voltage, and means for coupling said charging capacitor to said variable frequency oscillator means.

9. An automatic drive circuit for driving a mechanical displacement devicr or the like at resonance as set forth in claim 8,

wherein said sweep circuit means further comprises means for preventing discharging said charging capacitor in said oscillator circuit when such sweep control output signal indicates that such feedback voltage equals or exceeds such drive signal voltage.

10. An automatic drive circuit for driving a mechanical displacement device or the like at resonance as set forth in claim 9,

wherein said means for coupling in said sweep circuit means comprises a high impedance amplifier for amplifying the voltage on said charging capacitor, the output from said amplifier being coupled to said variable frequency oscillator means.

11. An automatic drive circuit for driving a mechanical displacement device or the like at resonance as set forth in claim 9,

wherein said sweep circuit means further comprises means for generating a minimum sweep signal to assure at least a minimum frequency for said output AC signal from said variable frequency oscillator means. 

1. An automatic drive circuit for driving a mechanical displacement device or the like at resonance, comprising in combination: a. variable frequency oscillator means for generating an AC signal at a frequency determined by an input signal thereto and in a range including the resonant frequency of such device; b. amplifier means responsive to such AC signal for producing a drive signal for said mechanical displacement device to effect displacement in the same; c. feedback means mounted to said device and responsive to such displacement for producing a feedback voltage proportionally representative of said displacement; comparator means for comparing the voltage of such drive signal and such feedback voltage, said comparator means producing a first sweep control output signal when said drive signal voltage exceeds such feedback voltage, and producing a second sweep control output signal when said feedback signal exceeds said drive signal; e. sweep circuit means having an input connected to the output of said comparator means and responsive to such sweep control output signal for generating a sweep signal as a periodcially varying voltage signal in response to said first sweep control output signal, when such drive signal voltage exceeds such feedback voltage, and for generating a fixed voltage output signal, when such feedback voltage equals or exceeds such drive signal voltage, said sweep circuit means having an output coupled to said variable frequency oscillator means to provide such sweep signal as such input signal to said variable frequency oscillator; whereby said voltage comparator means locks said sweep circuit means to produce a fixed voltage output signal at a magnitude to effect operation of said variable frequency oscillator means and amplifier means at a fixed frequency to provide a drive signal to the mechanical displacement device for driving the same at resonance when such feedback voltage equals or exceeds such drive signal voltage and whereby said voltage comparator unlocks said sweep circuit means to produce said periodically varying voltage signal to drive said variable frequency oscillator as a sweep frequency oscillator when said drive signal voltage exceeds said feedback voltage.
 2. An automatic drive circuit for driving a mechanical displacement device or the like at resonance, comprising in combination: variable frequency oscillator means for generating an AC signal at a frequency determined by an input signal thereto; amplifier means responsive to such AC signal for producing a drive signal for said mechanical displacement device to effect displacement in the same; feedback means responsive to such displacement for producing a feedback voltage proportionally representative thereof; comparator means for comparing the voltage of such drive signal and such feedback voltage, said comparator means producing a sweep control output signal indicative of whether such drive signal voltage exceeds such feedback voltage; sweep circuit means responsive to such sweep control output signal for generating a sweep signal as a periodically varying voltage signal, when such drive signal voltage exceeds such feedback voltage, and as a fixed voltage signal, when such feedback voltage equals or exceeds such drive signal voltage, said sweep circuit means being coupled to said variable frequency oscillator means to provide such sweep signal as such input signal thereto; whereby said voltage comparator means locks said sweep circuit means to produce such fixed voltage sweep signal at a magnitude to effect operation of said variable frequency oscillator means and amplifier means to provide a drive signal to the mechanical displacement device for driving the same at resonance when such feedback voltage equals or exceeds such drive signal voltage wherein said variable frequency oscillator means comprises a pulse shaping circuit.
 3. An automatic drive circuit for driving a mechanical displacement device or the like at resonance as set forth in claim 2, wherein said pulse circuit comprises a plurality of NAND circuits coupled to a JK flip-flop circuit.
 4. An automatic drive circuit for driving a mechanical displacement device or the like at resonance, comprising in combination: variable frequency oscillator means for generating an AC signal at a frequency determined by an input signal thereto; transistorized amplifier meAns responsive to such AC signal for producing a drive signal for said mechanical displacement device to effect displacement in the same; feedback means responsive to such displacement for producing a feedback voltage proportionally representative thereof; comparator means for comparing the voltage of such drive signal and such feedback voltage, said comparator means producing a sweep control output signal indicative of whether such drive signal voltage exceeds such feedback voltage; sweep circuit means responsive to such sweep control output signal for generating a sweep signal as a periodically varying voltage signal, when such drive signal voltage exceeds such feedback voltage, and as a fixed voltage signal, when such feedback voltage equals or exceeds such drive signal voltage, said sweep circuit means being coupled to said variable frequency oscillator means to provide such sweep signal as such input signal thereto; whereby said voltage comparator means locks said sweep circuit means to produce such fixed voltage sweep signal at a magnitude to effect operation of said variable frequency oscillator means and amplifier means to provide a drive signal to the mechanical displacement device for driving the same at resonance when such feedback voltage equals or exceeds such drive signal voltage wherein said amplifier means further comprises a coupling transformer coupled between the same and said mechanical displacement device, said coupling transformer including a center-tapped primary winding for periodic and alternate current flow in opposed directions through at least portions of the same, the secondary of said amplifier being coupled to provide such drive signal to drive the mechanical displacement device.
 5. An automatic drive circuit for driving a mechanical displacement device or the like at resonance as set forth in claim 4, further comprising an adjustable voltage power supply coupled to the center-tap of said coupling transformer primary winding for providing power thereto.
 6. An automatic drive circuit for driving a mechanical displacement device or the like at resonance, comprising in combination: variable frequency oscillator means for generating an AC signal at a frequency determined by an input signal thereto; amplifier means responsive to such AC signal for producing a drive signal for said mechanical displacement device to effect displacement in the same; feedback means responsive to such displacement for producing a feedback voltage proportionally representative thereof; comparator means for comparing the voltage of such drive signal and such feedback voltage, said comparator means producing a sweep control output signal indicative of whether such drive signal voltage exceeds such feedback voltage; k sweep circuit means responsive to such sweep control output signal for generating a sweep signal as a periodically varying voltage signal, when such drive signal voltage exceeds such feedback voltage, and as a fixed voltage signal, when such feedback voltage equals or exceeds such drive signal voltage, said sweep circuit means being coupled to said variable frequency oscillator means to provide such sweep signal as such input signal thereto; whereby said voltage comparator means locks said sweep circuit means to produce such fixed voltage sweep signal at a magnitude to effect operation of said variable frequency oscillator means and amplifier means to provide a drive signal to the mechanical displacement device for driving the same at resonance when such feedback voltage equals or exceeds such drive signal voltage wherein said feedback means comprises piezoelectric crystals coupled in said mechanical displacement device and wherein said comparator means comprises a coupling transformer having a primary winding connected for energization by said piezoelectric crystals in said feedback means, and a full-wave bridge rectifier coupled to the secondary winding of said coupling transformer fOr providing DC feedback.
 7. An automatic drive circuit for driving a mechanical displacement device or the like at resonance, comprising in combination: variable frequency oscillator means for generating an AC signal at a frequency determined by an input signal thereto; amplifier means responsive to such AC signal for producing a drive signal for said mechanical displacement device to effect displacement in the same; feedback means responsive to such displacement for producing a feedback voltage proportionally representative thereof; comparator means for comparing the voltage of such drive signal and such feedback voltage, said comparator means producing a sweep control output signal indicative of whether such drive signal voltage exceeds such feedback voltage; sweep circuit means responsive to such sweep control output signal for generating a sweep signal as a periodically varying voltage signal, when such drive signal voltage exceeds such feedback voltage, and as a fixed voltage signal, when such feedback voltage equals or exceeds such drive signal voltage, said sweep circuit means being coupled to said variable frequency oscillator means to provide such sweep signal as such input signal thereto; whereby said voltage comparator means locks said sweep circuit means to produce such fixed voltage sweep signal at a magnitude to effect operation of said variable frequency oscillator means and amplifier means to provide a drive signal to the mechanical displacement device for driving the same at resonance when such feedback voltage equals or exceeds such drive signal voltage wherein said comparator means comprises first and second isolation circuits and a comparison circuit, said first isolation circuit receiving such drive signal voltage and providing a first output signal representative thereof and said second isolation circuit receiving such feedback voltage and producing a second output signal representative thereof, said comparison circuit comparing said first and second output signals and providing said sweep control output signal as a high level signal when the former exceeds the latter and as a low level signal when the latter equals or exceeds the former.
 8. An automatic drive circuit for driving a mechanical displacement device or the like at resonance, comprising in combination: variable frequency oscillator means for generating an AC signal at a frequency determined by an input signal thereto; amplifier means responsive to such AC signal for producing a drive signal for said mechanical displacement device to effect displacement in the same; feedback means responsive to such displacement for producing a feedback voltage proportionally representative thereof; comparator means for comparing the voltage of such drive signal and such feedback voltage, said comparator means producing a sweep control output signal indicative of whether such drive signal voltage exceeds such feedback voltage; sweep circuit means responsive to such sweep control output signal for generating a sweep signal as a periodically varying voltage signal, when such drive signal voltage exceeds such feedback voltage, and as a fixed voltage signal, when such feedback voltage equals or exceeds such drive signal voltage, said sweep circuit means being coupled to said variable frequency oscillator means to provide such sweep signal as such input signal thereto; whereby said voltage comparator means locks said sweep circuit means to produce such fixed voltage sweep signal at a magnitude to effect operation of said variable frequency oscillator means and amplifier means to provide a drive signal to the mechanical displacement device for driving the same at resonance when such feedback voltage equals or exceeds such drive signal voltage wherein said sweep circuit means comprises an oscillator circuit including a charging capacitor and controlled means for substantially linearly charging said charging capacitor, said controlLed means being responsive to such sweep control output signal to effect charging of said charging capacitor when such sweep control output signal indicates that such drive signal voltage exceeds such feedback voltage, and means for coupling said charging capacitor to said variable frequency oscillator means.
 9. An automatic drive circuit for driving a mechanical displacement devicr or the like at resonance as set forth in claim 8, wherein said sweep circuit means further comprises means for preventing discharging said charging capacitor in said oscillator circuit when such sweep control output signal indicates that such feedback voltage equals or exceeds such drive signal voltage.
 10. An automatic drive circuit for driving a mechanical displacement device or the like at resonance as set forth in claim 9, wherein said means for coupling in said sweep circuit means comprises a high impedance amplifier for amplifying the voltage on said charging capacitor, the output from said amplifier being coupled to said variable frequency oscillator means.
 11. An automatic drive circuit for driving a mechanical displacement device or the like at resonance as set forth in claim 9, wherein said sweep circuit means further comprises means for generating a minimum sweep signal to assure at least a minimum frequency for said output AC signal from said variable frequency oscillator means. 